Installing an underground pressure tank

I'm getting ready to build a small summer home. I want the well water pressure tank to be underground, in part because of possible freezing. I've already bought the tank (fiberglass) and 4' of 36" pipe for the vault. Question is: How much plumbing must be in the vault?

I assume that I can put the pressure switch in the cabin (about 30' from the tank). Same with the pressure gauge. That leaves just the pressure relief valve, backflow preventer, and a drain valve in the vault, right?

The pressure tank I bought allows the bladder to be serviced from the top. No need to pull the tank for this operation. But if the tank does need to be pulled, are there some kind of disconnect flanges to help with this? Maybe something like the flanges on circulating pumps? I don't want to end up having to do complex plumbing in a vault that has limited space.

Your assumption is wrong. The pressure switch cannot be located remotely from the tank--it MUST be within 2', and I like it closer--on a cross tee made for this purpose taken right off the tank.
The 4' x 36" pipe will not give you enough room to work. I would encourage you to rethink this entirely. Unless you actually have a concrete pit large enough to get down into and work in, I would not put this in the ground (this is the voice of experience speaking and I am a fairly small person). If you install any yard hydrants off the main line between the well head and the pressure tank, you cannot install a check valve at the pressure tank location.
Deb
The Pipewench

The pressure tank vault that I remember looking into had the "T" at the top of the tank and about 6" from ground surface. From the T, a pipe ran down to two elbows that allowed connection to the bottom of the tank, but other than two elbows there's nothing to go wrong in the bottom of the vault and no reason for access. The vault was only bigger than the tank by enough room for the pipe.

I read on another site that the pressure switch doesn't need to be near the tank. Most say it has to be close. What's the reason for needing it near the tank? I was hoping to for remote placement since a vault isn't a very friendly place for electrical contacts.

I found one solution to pressure tank removal from the vault. Hot water heaters connect with 3/4" flexible pipe and allow the HWH to be easily replaced. I could use the same system for the pressure tank, but I don't think 3/4" would be good idea for handling all of the water in the house. Falcon makes stainless lines in the standard 3/4", and also in 1", 1.5, etc. 3/4" is $6 and 1" is $25 (ouch) , but probably worth having the extra capacity.

The pressure tank with the T already plumbed could be lowered into the vault. From then on, all connections and plumbing could be done from the surface.

IMHO, a vault is not a good place for a pressure tank either.
I live and work in an area where all water is supplied by private or small neighborhood well systems. I would never put a pressure tank in a vault like you have suggested.
It and/or the fittings (which should all be brass) will at some point fail and need to be replaced.
You cannot use flex connectors for an application like this.
Where will you install the drain and how will you access it?
Where do you live and how cold does it get? How deep are you required to bury your water lines? The top of the tank can be no higher than that. You will also most likely need some other type of insulation in your vault, since there is no earth over the top of the tank to keep it from freezing.
Have you made provisions so that rain water and ground water will not collect in the vault and submerge everything?
I could go on and on...
I have never heard or read anywhere that the pressure switch can be located remotely from the pressure tank. Could you post back where you read this, I would be interested in reading it, also. Simply put, the tank and switch work too closely together to be located remotely from each other. What do the instructions that came with your tank and/or switch say?

I have to say that I believe that this idea is fraught with problems and potential problems. I'm not sure that I would even install a pressure tank like this for someone I didn't like and I really urge you to rethink things. All plumbing should be installed knowing that one day it will need to be replaced or repaired. Well water is especially bad.

I'm just outside Seattle, so there's really no need to go deep to avoid frost. Our code says water pipes in unconditioned space must be placed "to avoid freezing." Foundation footings are required to be 12", but anything just under the surface is enough to prevent pipe freeze. A really big cold snap for us is 5 days in a row at 28 degrees, getting down to 20 at night. It can freeze pipes and toilets in an unheated house, but the frost depth isn't more than an inch. In fact, I've never seen frozen soil here.

The vault I saw in Eastern Washington, where they do get a good freeze, had the top of the tank close to the surface, but the depth of the vault kept it from freezing. It's been in place for several years.

I've got a couple of other reasons for wanting the tank outside and underground besides just the freezing issue. The house is really small and space is at a premium. The building site is very small and I don't want a pumphouse in the yard that I have to worry about the temperature inside (this is all of my neighbors' solution). Also, I love the cold, cold water coming out of the well. If it sits in a pressure tank in conditioned space, a glass of cold water is 70 degrees.

The tank I bought is a fiberglass 35 gallon with a stainless elbow. Light as a feather when empty. If I had to, I could get my backhoe to the vault and pull it out even if full. Drainout would be on the down tube to the tank. Vault would be set in glacial till with a pea gravel bottom. The soil drains like crazy, but it might take 20 minutes to get rid of the 10 gallon draw down if the tank had to be pulled for some reason. The vault has a waterproof top and pipe entries. It is manufactured by Orenco, a big name in tanks, vaults, septic systems, etc. Surface water shouldn't be a problem.

I'll look for the site that said a remote placement of the pressure switch was okay. I bought a Square-D Pumptrol from a plumbing supply house and it didn't have any instructions in it other than a wiring diagram inside the housing. Kind of surprised me that there wasn't further instructions. I'll check the SD website.

Why not use corregated stainless connections to a well pressure tank? They work fine for the HWH. Same system, same pressure, etc. Simplifies replacement and a pressure tank seems to periodic replacement just like the HWH.

Very close to Seattle doesn't get that cold. In Summerset on the hill above Bellevue I've seen it freeze at least six inches.

North of that in Millcreek, I've seen it freeze close to a foot.
I was doing ground works that day and shattered two wooden pick handles getting past the top layer of frozen earth.
It was cold for a month that year. The outfit I was working with sent 50 plumbers home that month. I worked.

I got ahold of the person who has the underground pressure tank near Mazama, WA that I had seen. It's at a cabin used for cross-country skiing. The frost depth there is 5'. The pressure tank vault is 7' deep and includes the pressure switch. The installer said that he had done this for 17 years and never had one freeze or a pressure switch problem. That extra two feet was enough to keep the air from getting cold enough to freeze the tank.

The installer said he put a little grease on the electical contacts and that's all. Of course, the ground is drier on that side of the mountains. I'm still spooked about an underground pressure switch.

As to a remote pressure switch, here's what I've learned. The problem is with the loss of head in the pipe between the pressure tank and the pressure switch. If the switch is 100' from the pressure tank, and there are lots of couplings and 90 degree elbows in the supply pipe, and the pipe is only 3/4", and the pressure switch is 10 feet above the tank, and you're trying to run 6 gpm to the house, you've got a problem because of the loss of head caused by these things. Loss of head equates to loss of psi (2.31 foot of head = 1 psi), which can equate to inaccurate pressure settings.

Let's say the loss of head is 10 psi between the tank and a remotely mounted 40-60 psi switch. (This number is huge, but easy to work with). You have a pressure gauge at the tank and one at the switch. The bladder in the pressure tank is at 38 psi, 2 lbs less than the 40 psi cut in pressure.

But because of the head loss, when the remote switch kicks off the pump at "60 psi," the tank might actually have 70 psi in it, because it doesn't suffer from the subsequent loss of head. After the pump kicks is off, the pressure gauge mounted at the switch would continue to rise, showing that the system is equalizing somewhere between 60 and 70. Probably closer to 70. The gauge at the pressure tank would drop as the pressure equalizes.

When the pump comes on again at "40 psi," the pressure gauge at the tank might still read 50 psi. This inaccuracy effects both the drawdown amount and the pressure that you should have in the tank bladder. If your bladder pressure is set for the "standard" 2 lbs less than than the cut in pressure on the switch, it is set wrong. The bladder either needs to be set at 48 psi (because 50 is the real cut in pressure) or the pressure switch needs to be adjusted so that it cuts in at 40 psi at the tank. This difference in the pressure at the tank and the pressure sensed at a remote switch can be a problem. It can reduce your drawdown amount so that your pump is cycling. Just what the pressure tank and switch are supposed to stop.

You generally don't know and don't care about the minor differences in pressure caused by head loss thoughout the system. If you have 40-60 psi at the pressure tank, that's good enough for most situations. You don't care that the pressure actually hits 85 psi at the well head or that it actually is 31-51 psi in the third floor bath.

You can still have 40-60 psi at the tank with a remote mounted pressure switch if you adjust the pressure switch based on the reading at the tank, not at the switch. You can reduce the head loss (always a good thing) with larger pipe, less distance, fewer fittings, etc.

Most installers probably don't want to get involved with yelling back and forth from the house to somebody at the pressure tank in the yard. But I my research has shown that nothing bad, like water hammer, legionaire's disease, etc., is caused by a remote mounting of the switch. It's primariliy an ease of installation issue.